The present invention relates to components containing elemental carbon, including components containing graphite, amorphous carbon and carbon fibers, and components containing Carbon-Carbon (hereinafter “C—C”) composites.
C—C composites possess a combination of high strength, high fracture toughness, low density, very high thermal conductivity and high electrical conductivity. The mechanical strength of C—C composites increases as operating temperature increases, in sharp contrast to most other materials, including metals, metallic alloys and ceramics, which become softer and weaker as the operating temperature increases. This combination of attributes makes C—C composites good candidates for high temperature applications such as aerospace heat exchangers and aircraft brake pads.
However, the carbon in the C—C components tends to oxidize when exposed to air or other oxidizing environments at temperatures exceeding approximately 300° C. When the carbon is oxidized, it loses mass. This loss in mass directly leads to loss of mechanical strength of the component, as well as loss of integrity, functionality and ultimately failure of the component.
Barrier coatings such as SiC and SiO2 (silica glass) may be applied to the components in order to protect the carbon from oxidizing when subjected to sustained or repeated high temperatures. Both SiC and SiO2 prevent and/or retard oxidation when free of microcracks and other defects because the oxidizing species must diffuse or filter through these coating materials to reach the underlying surface of the C—C component. Such diffusion is negligibly slow at temperatures below about 800° C., for both dry oxygen and steam oxidants. Also, the chemical reaction of SiC with oxygen to produce SiO2 is negligibly slow below about 800° C.
SiO2 has a much lower mechanical strength than that of SiC, leading in principle to earlier failure of the SiO2 coating when the temperature is below its softening temperature (about 1150° C.). SiO2 has lower density (2.2 g/cm3) than SiC (3.2 g/cm3).
Barrier materials such as SiC, for example, when applied to C—C and other carbon-containing components, do not afford complete protection against oxidation due primarily to the difference in thermal expansion coefficients (TCEs) between the specific coating material and the carbon-containing component. Such barrier coatings are usually applied at a relatively high temperature from vapor-phase or liquid-phase precursors. After application of the coating material, the coated component cools down to room temperature. Due to the difference in TCEs between the coating material and the carbon-containing component, high stresses develop in the coating, which lead to microcracks throughout the coating. Many of these microcracks in the coating reach through the thickness of the coating to the coating-carbon interface. Some microcracks may also develop in the underlying carbon-containing component. During subsequent exposure to an oxidizing ambient at a temperature higher than 300° C., oxidants may penetrate through such microcracks into the underlying carbon-containing component and undesirably oxidize the carbon therein. In addition, during repeated thermal cycles experienced by the coated component throughout its useful lifetime, additional microcracks may develop in the coating, as well as in the underlying carbon-containing component.
Further, the density of microcracks usually increases with the difference in TCEs between the two materials and with the coating application temperature, due to the corresponding increase in the stress generated in the coating.
Protecting carbon-containing components from oxidation is particularly troublesome when the component is very thin, for example when the thickness is 3 to 60 mils (0.07-1.5 mm), and/or when the component has a complex shape. For example, very thin-gauge and/or complex-shaped carbon-containing components are used in C—C heat exchangers for operation in the temperature range of approximately 25 to 800° C. for total times of about 8,000 to 20,000 hours. Such a component may not be allowed to lose more than a small fraction of its original weight, for example about 1% to 5% of the weight, without measurable and/or serious degradation in its functional properties, such as mechanical strength (in contrast, a relatively thick, for example, 1 inch or 25 mm thick, C—C component used as a disk brake pad in an airplane or as part of a missile may lose 10 to 30 percent of its initial weight without failing its functional mission).